It is well known that the combustion of solid fuel, such as coal, produces fine particles of non-combusted materials which are carried out of the fuel bed by the draft. These particles are typically called fly ash. In large installations, such as power plants, the fly ash is collected by means of electrical precipitators which are known to the prior art. Briefly, electrical precipitators have negative electrodes and grounded collecting plates. A negative charge is placed on the fly ash particles by the negative electrodes causing them to be attracted to the collecting plates. In theory, contact of the charged particles with the collecting plates neutralizes the particles causing them to fall from the collecting plates to a point where they are vacuumed into a storage tank. Separation of the particles from the collecting plates may be assisted by vibrating the collecting plates.
It has been recently determined that cenospheres are a major constituent of fly ash. The fly ash cenospheres are chemically inert hollow spheres having a diameter up to approximately 300 microns. They are composed chiefly of silicon oxide, aluminum oxide and iron oxide with nitrogen in their void. Heretofore, such spheres have been made commercially, principally for use in reflectors and as buoyancy devices in deep diving submarines. The high cost of their manufacture has severely limited their application to other uses. However, the cost of fly ash cenospheres is only that of separating from the other fly ash constituents and, with an efficient system for separating and grading fly ash cenospheres, many other uses are envisioned. For example, cenospheres may be employed as heat insulators and fillers for plastics and rubber. Other cenosphere uses are being examined.
The present method of separating fly ash cenospheres is to place the fly ash in holding ponds where the cenospheres rise to the surface because of their buoyancy. They are then dried and marketed. However, the cenospheres obtained in this manner include dust particles as well as small carbon black particles. In examining the residue at the bottom of the holding ponds it has also been determined that a great portion of the cenospheres are trapped at the bottom of the ponds. It is believed that this results from the fact that the fly ash collected with electrical precipitators is highly charged causing the cenospheres, dust and carbon black to cling together in clusters causing many of the cenospheres to sink while those that do not sink have dust and carbon black particles clinging to their surface.
The highly charged character of the fly ash has been established by examination of the fly ash prior to the placing it in the holding pond. Many factors are believed to contribute to this charge. For example, a build up of fly ash on the collecting plates reduces the ability of the collecting plates to electrically neutralize the fly ash. Also, movement of the fly ash particles relative to each other produces a static electricity build up in the particles while the flue gases passing through the collecting system ionizes those gases bringing about coulombic forces.